1. Field of the Invention
This invention relates generally to a process and apparatus for the removal of organic sulfur compounds from a stream containing hydrocarbons. In particular, it relates to the use of an aqueous caustic solution to remove mercaptans from a hydrocarbon stream.
2. Description of the Related Art
It is conventional practice at the present time to treat sour gas and liquid hydrocarbon streams to remove mercaptans. Extraction processes are typically used when treating light liquid hydrocarbons and gas streams for mercaptan removal. Mercaptans have traditionally been removed from hydrocarbon streams because of their malodorous scent.
U.S. Pat. No. 5,244,643 discloses a process whereby a hydrocarbonaceous gas stream including mercaptan sulfur, air and aqueous alkaline solution including a mercaptan oxidation catalyst are mixed in a mixing vessel in which mercaptans are converted to disulfides. The effluent withdrawn from the top of the mixing vessel is settled in a vessel to yield separated streams of air, liquid hydrocarbon product containing disulfide, and an aqueous alkaline solution including mercaptan oxidation catalyst.
U.S. Pat. No. 4,562,300 discloses contacting a hydrocarbon stream including organic mercaptans with sodium hydroxide to free the hydrocarbons from the organic mercaptans. The caustic solution rich in mercaptans is oxidized with a catalyst and the organic mercaptans are converted to organic disulfides. The mixture of regenerated caustic solution free of organic mercaptans and organic disulfides enter a settler in which the organic disulfides and caustic solution are separated. A stream of hydrocarbon from which the mercaptans had been extracted upstream are admixed with regenerated aqueous caustic solution containing small amounts of organic disulfides to extract the organic disulfides from the regenerated caustic solution.
In a typical liquid-liquid extraction process, a liquid hydrocarbon stream is fed to an amine absorber column to be contacted with an amine, such as diethylamine, to absorb acid gases such as hydrogen sulfide and carbon dioxide from the hydrocarbon stream. The hydrocarbon stream lean of hydrogen sulfide and other acid gases is prewashed in a prewash vessel containing 6.5 to 15.0 wt. % liquid caustic to convert the remaining hydrogen sulfide to sodium sulfide which is soluble in caustic. The hydrocarbon stream, now depleted of hydrogen sulfide, is subjected to counter-current flow of a liquid caustic in an extraction vessel. Jet decks in the extraction vessel facilitate the counter-current contact. Mercaptans in the hydrocarbon stream react with the caustic to yield mercaptides. The mercaptides in the hydrocarbon stream are soluble in the caustic. A product hydrocarbon stream lean in mercaptans passes overhead from the extraction column through a settler drum to a sand filter vessel, and the mercaptide rich caustic passes from the bottom of the column. The settler drum allows for settling and buffers the sand filter against caustic surges. The sand filter coalesces caustic to make it gravitate to the bottom of the vessel while the product hydrocarbon stream passes out of the vessel through an outlet that is shielded at the top to prevent admittance of falling caustic droplets. The mercaptide rich caustic receives an injection of air and catalyst as it passes from the extraction column to an oxidation vessel for regeneration. Oxidizing the mercaptides to disulfides using a phthalocyanine catalyst regenerates the caustic solution. A disulfide separator receives the disulfide rich caustic from the oxidation vessel. The disulfide separator vents excess air and decants disulfides from the caustic before the regenerated caustic is drained and returned to the extraction vessel. The disulfides are run through a sand filter and removed from the process.
One conventional mercaptan extraction system utilizes four vessels: the caustic prewash vessel, the extraction vessel, the settler drum, and the sand filter vessel. Each vessel must have its own support and base structure making building this system capital-intensive. U.S. Pat. No. 6,749,741 discloses combining the extraction vessel, the settler drum, and the sand filter vessel into one vessel to thereby reduce the capital required to build an extraction system. In the combined vessel, a caustic solution enters the extractor section of the vessel and passes through this portion of the vessel in order to remove mercaptans from a hydrocarbon stream resulting in spent caustic rich in mercaptides. The spent caustic is then recovered from the extractor section of the vessel and regenerated in a downstream process. The regenerated caustic can then be recycled back to the extractor section of the vessel. Furthermore, spent caustic in the prewash section of the vessel that is rich in sulfide salts can be recovered. Part of the recovered caustic is recycled back to the prewash section of the vessel while the remainder is removed from the process.
In the above described mercaptan extraction system, the caustic is regenerated to allow reuse in the extraction vessel, thus limiting the amount of caustic needed. However, the need for a caustic regeneration necessitates additional capital to build a regeneration system.
Accordingly, what is needed is a “once-through” type extraction vessel, where the caustic is only used once before being sent to effluent treatment to thereby further reduce the capital required to build an extraction system. Also, there is a need for a “once-through” type extraction vessel which minimizes the amount of fresh and spent caustic that is needed.